The invention relates to the field of predistorters and linearization of high power amplification. More particularly, the present invention relates to feedback complex processing of input signals for improved linearization of high power amplifiers.
Satellite systems have applications from television broadcasts to military communications requiring reliable transfer of information from one location to another. All satellite systems incorporate the use of high power amplifiers (HPA) to boost signal strength for transmission. For example, HPAs are used by the Global Broadcasting Service to boost the signal strength of transmitted data between satellites and ground-based stations. When using satellites for ground based communications, the HPAs provide the transmitted signal with ample signal strength to account for signal attenuation due to path loss and other disturbances. Amplifying the input signal to levels required for reliable transmission using currently available HPAs, such as solid state power amplifiers or traveling wave tube amplifiers, introduces amplitude distortion and phase distortion. HPAs are nonlinear devices that induce distortion of the original signal before transmission. Nonlinear operation of the HPAs operating in the saturation region at point of maximum power transfer induces the distortion. Both the amplitude and phase of the original input signal are distorted compromising the integrity of the output signal. Because the HPAs are nonlinear devices, amplification of an input signal results in amplitude modulation to phase modulation (AM-PM) distortion and amplitude modulation to amplitude modulation (AM-AM) distortion of the output signal. The amplitude and phase distortion of the HPAs results in a distorted high power output signal. It is desirable to reduce the amplitude and phase distortion for continuous reliable communications. The amplitude and phase distortion can cause intermodulation components and spectral regrowth, which are undesirable effects leading to poor communications. For example, intermodulation components and spectral regrowth can cause adjacent channel interference to other communication services in addition to a loss in transmit power. The effects of amplitude and phase distortion can cause signal distortion that can degrade bit error rate (BER) performance during communications. The amplified signal exhibits undesirable spectral regrowth characterized as added signal spectra outside of the desired bandwidth with an additional increase in the BER.
One method to reduce the undesirable effects of the amplitude and phase distortion is to predistort the input signal so as to linearize the HPAs output with the input signal so that the resultant amplified output signal has reduced amplitude and phase distortion and the power spectrum remains within the communication bandwidth without spectral regrowth. Predistortion involves using known properties of HPAs in order to provide input data to an algorithm that predistorts the original signal. When the predistorted input signal is passed through the HPAs, the power spectrum of the transmitted output signal is nearly identical to that of the original signal before predistortion and amplification. In essence, the predistorter corrects for the nonlinear behavior of the HPAs so that the resulting amplification appears as a linear process.
Previous methods to predistort the original input signal use a Saleh model, an extended Saleh model, a linear-log model, and a modified linear-log model. The use of each of these methods disadvantageously requires advance knowledge of the characteristics of the HPA with the predistortion compensating for distortion effects. These predistorter models can correct for nonlinear behavior exhibited by HPAs to varying degrees. These models rely on an erroneous assumption that the HPAs performance will remain constant during the life of the HPAs. In reality, however, the HPAs are subject to change as the HPAs do not have constant characterizations. The use of each of these models disadvantageously requires a priori known characteristics of the HPAs in order to design a predistorter that then disadvantageously does not change over time. Therefore, these predistorter models are effectively nonadaptive to changing HPA characterizations. The problem inherent with these nonadaptive models is a lack of adaptive predistortion to account for changes in the HPA characteristics. The characteristics of the HPAs may change with any number of known or unknown variables such as age or ambient temperature. As a consequence, these nonadaptive predistortion methods disadvantageously provide nonlinear results under varying HPAs operating conditions and changing HPA characteristics. A more desirable solution is a predistorter that linearizes the HPAs over changing HPA characteristics.
Because the transmitted signal may be communicated at a very high carrier frequency, at least on the order of megahertz, signal processing at the modulated frequency requires a very costly high speed signal processing system. Additionally, algorithms that mitigate the distortion can not be executed on board a satellite due to the need to first characterize the HPA performance on the ground. As a result, the nonadaptive predistortion methods disadvantageously perform the signal processing at high speed using expensive signal processing systems. The prior predistorter models are disadvantageously implemented in high speed processes that are statically configured on the ground for known static HPA characteristics and hence are nonadaptive. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a system for linearizing the output of a high power amplifier.
Another object of the invention is to provide a system for linearizing the output of a high power amplifier through predistortion.
Yet another object of the invention is to provide a system for linearizing the output of a high power amplifier through adaptive predistortion.
Another object of the invention is to provide a system for linearizing the output of a high power amplifier through complex adaptive predistortion.
Another object of the invention is to provide a system for linearizing the output of a high power amplifier through complex adaptive predistortion using an adaptive predistorter operating at baseband.
Still another object of the invention is to provide a system for linearizing the output of a high power amplifier having changing characteristics.
The invention is directed to a system that predistorts an original input signal to compensate for undesired amplitude and phase distortion while reducing spectral regrowth during high power amplification. The input signal is predistorted so that the resultant output signal, when amplified, has the same power spectrum as the original input signal after baseband filtering. The predistorter functions to linearize the operation of the high power amplifier (HPA) having a resultant one-to-one spectral correlation between the input and output power. The system provides adaptive predistortion of the input signal for an HPA with unknown or varying characteristics.
The adaptive predistorter uses closed-loop feedback of the HPA output signal and the original input signal for controlled adaptive changing of the amount of the predistortion and hence adaptively operates to linearize the HPA output in the presence of changing HPA characteristics. The closed-loop feedback operates on the input signal and output signal in a complex form and at baseband. The improved adaptive predistorter uses the changing characteristics of the HPA as defined by signal measurements and continuously adapts the amount of predistortion of the original input signal to compensate for the nonlinear distortion. The adaptive predistortion processing algorithm can be implemented on-board a satellite. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.